Don't forget to fill the bathtub up with water in the event of a impending disaster. Would be used real good for flushing the toilet
As far as the water heater goes sucking out of the top would be better but remember this is survival. I don't think it going to harm you to much. When you use hot water it pulls it off the top of the tank and the cold water has a drop tube the brings in the cold h2o on the bottom. That in its self will stir up the water and mix up a lot of sediment and such.
They even make Cyclone water heaters for restaraunts that mix up all the water and keep sediment from collecting.